Solvents acid/base behaviour

M. M. Davis, Acid-Base Behaviour in Aprotic Organic Solvents, NBS Monograph No. 105, National Bureau of Standards, Washington, DC, 1968, p. 9. 

King, E. J., Acid-base behaviour, in Physical Chemistry of Organic Solvent Systems (Eds A. K. Covington and T. Dickinson), Chap. 3, Plenum Press, New York, 1973. 

Solvent effects on the dissociation of 11 2,6-disubstituted benzoic acids have been analysed by chemometric analysis.66 The acid-base behaviour of the three zwitterionic pyridinecarboxylic acids (picolinic, nicotinic, and isonicotinic acid) has been studied. The cationic form of picolinic acid converts partially into the corresponding zwitterion within a borderline acidity range (pH/acidity function). The various pXa values were determined for the three isomers by spectrophotometric and potentiometric methods and reasonable agreement was found.67... 

Classification of Solvents in Terms of Acid-Base Behaviour 73... 

Classification of Solvents in Terms of Acid-Base Behaviour 75 Table 3-5. Classification of organic solvents according to their Bronsted acid-base behaviour [56]. 

Recent gas-phase studies of proton-transfer reactions with stepwise solvation of the reactants i.e. incremental addition of solvent molecules to form supermolecular clusters) have demonstrated that the acid/base behaviour of isolated solvent molecules can be dramatically different from their performance as bulk liquids. Water, the classical amphiprotic solvent, shall serve as an example. 

King, E. I., Acid-base behaviour, in Physical Chemistry of Organic Solvent Systems (A. K. Covington and T. Dickinson, eds.). Plenum Press, London, 1973, pp. 331-403. Kolthoff, I. M. and M. K. Chantooni, General introduction to acid-base equilibria in nonaqneons organic solvents, in Treatise on Analytical Chemistry, Parti, Theory and Practice (I. M. Kolthoff and P. J. Elving, eds.), John Wiley Sons, New York, 1979, pp. 239-301. 

This result, like that found for sulfur dioxide, is somewhat ironic, as phosgene played an important rdle in the development of Franklin s theory of the acid-base behaviour of non-aqueous solvents [736,738a,850d,1285a], There has been a very recent suggestion, however, that [COCl]" is stabilized by a zeolite framework [641], and exhibits r(CO) at 1790 and 1710 cm [641], This claim requires substantiation, however. 

The self ionisation of H20(1) must always be considered when acid/base behaviour in aqueous solution is discussed. For instance, a weak acid in aqueous solution is a source of H30+(aq), but so also is the solvent, H20(1) a weak base in aqueous solution is a source of... 

In contrast, the salt formed by the neutralisation of methanoic acid, HCOOH(aq), with NaOH(aq) gives a basic solution with the pH dependent on the concentration of the salt. Sodium methanoate is the salt of a weak acid and strong base, and in the solid consists of Na and HCOO ions. When dissolved in water, Na" "(aq) ions show no acid/base behaviour with the water since NaOH(aq) is a strong base consisting of Na" (aq) ions and OH (aq) ions only. However, HCOO (aq) is the conjugate base of the weak acid HCOOH(aq), and will thus accept a proton from the solvent water to form the undissociated acid HCOOH(aq) in the... 

Acid-base behaviour in non-aqueous solvents Bromine trifluoride... 

In continuation of their studies of conformational equilibria and acid-base behaviour of dicarboxylic acids as a function of solvent, Roberts and co-workers have published the results obtained for 2-carboxy-l,4-butanedioic acid and its mono-, di-, and trianions in dimethyl sulfoxide which were investigated exclusively by the use of Vhh couplings. 

Non-hydrogen-bonded liquids of high dielectric constant are often referred to as dipolar aprotic solvents. Nitrobenzene, V,V-dimethyl-formamide, acetonitrile, and nitromethane have dielectric constants in the range 35 to 38, close to that of methanol. The dielectric constants of sulpholane, dimethylsulphoxide and propylene carbonate are higher. Any peculiarities of acid-base behaviour in dipolar aprotic solvents as compared with methanol cannot be accounted for on the basis of the Born sphere-in-continuum model. 

Useful exploratory studies of acid-base behaviour in solvents of low dielectric constant have been made by conductance " and potentio-metric " titrations. Association constants are usually obtained from spectrophotometric measurements. The strengths of various bases can be compared by means of their association with an indicator acid like 2,4-dinitrophenol. If both acid and base are colourless, a competition for the base can be established between the acid and an indicator acid like bromophthalein magenta In solvents like benzene, other reactions than simple 1 1 association between B and RX may occur. Self-association of the acid or base is one such auxiliary reaction. A classic example is the dimerisation of carboxylic acids in benzene. If allowance is not made for this, constant values of the quotient [BRX]j[B][RX] will not be obtained. (Variations in the quotient cannot be attributed to interionic forces or other nonideal behaviour BRX is scarcely dissociated into ions at all and in spectrophotometric work very low concentrations of B and RX can be used.) Evidence for association ratios other than 1 1 can be obtained from indicator studies. The method developed by Kolthoff and Bruckenstein for studying reactions in anhydrous acetic acid fails for reactions in benzene and similar solvents because more than one acid molecule reacts with the indicator to give complexes of the form/w J r"(HX)yi. In such studies it is generally a good approximation... 

For a large number of organic functionalities, significant protonation is only achieved in more concentrated acid solutions e.g. alcohols, ethers, ketones, esters, sulfides, sulfoxides). More concentrated acid solutions cannot be treated as ideal, and Ka values cannot be measured in terms of concentrations as in eqn (3.4). In strong acid media, the significantly decreased water concentration results in additional solvent effects on pA"a that are not accounted for by the pH scale. To account for acid-base behaviour in strong acid media, a number of acidity functions have been established. One of the earliest examples was the Hammett Ho acidity function based on a pairwise comparison of spectrophotometric changes in a series of aniline bases in concentrated acid solution. However, this scale could only be applied for structurally similar bases with similar protonation behaviour. Several other acidity functions have been proposed for other classes of bases such as the Hr acidity function for the ionisation of alcohols. As recently reviewed by Scorrano and More O Ferrall, later treatments by Bunnett and... 

King, E.J. Acid-base behaviour. In Physical Chemistry of Organic Solvent Systems, Covington, A.K., Dickinson, T., Eds. Plenum Press London, 1973 331 103. 

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